24 ROMANCE OF THE INSECT WORLD CHAP. 



form of two longitudinal cords, extended from the 

 tail end of the body along the median line of the 

 under-surface, parallel with each other and close 

 together, excepting at their anterior part, where they 

 separate and pass upwards to encircle the gullet be- 

 tween them, re-uniting above it in the head. Each of 

 the cords possesses a series of enlargements, or 

 ganglia, throughout its course, situated at certain 

 distances apart, their number corresponding to the 

 number of the larval segments. They are precisely 

 similar in position in both cords. Though appar- 

 ently a simple structure, each cord is in reality 

 composed of two distinct but closely united columns 

 of fibres, placed one upon the other. The under or 

 external column, which is nearest to the exterior of 

 the body, alone bears the ganglia, the upper one, 

 next to the digestive organs, is entirely wanting in 

 knots. These cords together are believed to repre- 

 sent the cerebro-spinal system of vertebrates. The 

 ganglion-less internal column of either cord is prob- 

 ably analogous to the motor nerves of the higher 

 animals, and the external column which is possessed 

 of ganglions,* answers to the sensitive nerves of the 

 vertebrata. Thus the two cords are each composed 

 of a motor and a sensitive column. The double 

 ganglia in the head, situated over the gullet, are 

 the largest, and represent the brain. These and the 

 ganglia previously described regulate the animal life 

 of the insect. The vegetative life beyond will-control 

 is maintained by an off-shoot from the principal 

 nervous structures. 



Just as the body segments of the larva coalesce in 



